The measurement of the mass or amount of particles, as dust, powders, dirt, smoke, fine liquids and solid aerosol particles, moving with a fluid, as air, can be accomplished with devices using visual, audio, or electrical parameters. The detection of particles moving in a passage has been accomplished with the use of structure as a probe or screen extended in the passage. These structures interfere with the flow of fluid and material in the passage and also can cause blockage of the passage. Examples of interfering structures in passages are shown by Worswick in U.S. Pat. No. 3,068,696 and Gosbell in British Pat. No. 1,184,073.
Electro-mechanical sensitive material has been used to detect pressure waves resulting from kinetic energy inside the wall of a pipe. Gibney shows in U.S. Pat. No. 2,936,619 a pipe having a plurality of serrations and a transducer which senses the frequencies of the liquid flowing in the pipe over a series of serrations.